The present invention relates generally to production of distortion-free photographic maps, known as orthophotographs, which are orthogonal projections of the ground on a horizontal plane. The invention relates more particularly toward instrumentation for producing orthophotographs by generating digital profile information from a stereoscopic model, storing the digital information on magnetic tape, and using the information in an offline mode to control the drive systems of an orthophotograph printer.
An orthophotograph, as known in the prior art, is a photographic map of a terrain with every detail ideally represented in its true horizontal position with any distortion caused by camera parallax and terrain relief corrected. The basic procedure for producing an orthophotograph involves creating a pair of aerial photographs of a terrain taken at different angles to formulate a three dimensional, optical model of the terrain. The two photographs (transparencies) are respectively placed in each of two projectors and are projected to create the three dimensional optical model by viewing the superimposed images through appropriate light filters or an image alternator. A photosensitive sheet is exposed to one of the transparencies through an aperture that scans the sheet, exposing only a small portion at a time. As the sheet is scanned, the height of the photosensitive sheet is adjusted by the operator so that the portion of the sheet being exposed is always kept on the apparent ground plane of the optical model.
Apparatus for exposing the photosensitive sheet so as to produce an orthophoto negative in the above described manner is disclosed in U.S. Pat. No. 2,869,419 to Bean, assigned to the assignee of the present invention.
While generally satisfactory, the above described system requires constant attention by the operator to maintain contact between the exposure aperture and the apparent terrain surface of the stereo model as the aperture scans across the model at constant speed. This type of system produces extreme operator fatigue with no provisions for correcting operator error. In addition, the system produces only single orthophoto negatives and is not adaptable for larger scale production or for editing.
Another type of system for producing orthophoto negatives, such as Stereomat, manufactured by Raytheon Autometics, includes image correlation devices that utilize a cathode ray tube to automatically correlate conjugate imagery and expose it on film in orthophotographic form. Such systems have high cost, and there is a likelihood of correlating unwanted images, such as treetops, which are not on the ground datum.
In another approach, incorporated in an apparatus known as the Gigas-Zeiss, manufactured by Zeiss Division of Keuffel & Esser, line-drawn profiles are obtained mutually by scanning a stereo model and then by electronic following techniques. These profiles are scanned to create output signals that are used to control drive motors in an automated exposure instrument. This type of instrumentation is expensive, and lacks versatility since it cannot be adapted to accomodate the wide variety of photomap products currently demanded.
Finally, semiautomated digital systems are presently available, wherein terrain data are generated in the same manner as the line-following technique, described above, except that the output is a digital record rather than a series of profile line drawings. The output data can be in the form of punched paper tape, magnetic tape or any other digital storage medium. Such a system is the K-320 manufactured by the Danko Arlington Company. While this type of system is more versatile than the systems described above, the equipment is extremely complex, and therefore quite costly, and requires the purchase of dedicated stereo plotters and orthophoto instruments.
There still exists a need for instrumentation that can be used in connection with a variety of different stereo plotters and orthophoto exposure instrumentation, and which can be adapted to accomodate various orthophoto applications, as required.
Accordingly, one object of the invention is to provide a new and improved system for producing orthophotographs.
Another object of the invention is to provide a new and improved system for producing orthophotographs useable with any of a variety of non-dedicated stereo plotters and orthophoto exposure instruments.
Another object is to provide a new and improved system for producing orthophotographs, wherein digital profile data are generated by a stereo plotter during scanning of a stereo model, which data are subsequently used to control an orthoprinter.
Yet another object is to provide a new and improved system for producing orthophotographs, wherein digital profile data are generated by a stereo plotter during scanning of a stereo model, and the digital data are scaled and stored in a memory to be subsequently used to control the drive members of an orthophoto exposure instrument.
Still another object is to provide a new and improved system for producing orthophotographs, including a control apparatus interfacing a stereo plotter and orthophoto exposure instrument, wherein the digital profile for each line scan of a stereo model generated by the plotter is stored on a magnetic tape as a single block of information for subsequent use in controlling an orthophoto exposure instrument.
Still another object is to provide a new and improved system for producing orthophotographs, wherein profile data for each line scan of a stereo model made by a stereo plotter are digitally encoded and stored on magnetic tape for subsequent use in controlling an orthophoto exposure instrument, wherein information associated with each line scan may be modified or edited as required, without affecting the remainder of the orthophotograph.
Another object of the invention is to provide a new and improved system for producing orthophotographs, wherein each line scan of a stereo model made by a stereo plotter is digitally encoded and stored on a magnetic tape as a starting coordinate and a series of incremental coordinate data to be used for controlling the drive members of an orthophoto exposure instrument.
Yet another object is to provide a new and improved system for producing orthophotographs, wherein terrain slope is continuously monitored during scanning of a stereo model, and the operator is alerted on occurrence of excessive slopes that may arise as a result of operator error or in any event would exceed system capabilities, and wherein slope correction signals are developed during playback to be supplied to conventional slope correction optics.